The growing use of data converters has created a demand for more accurate and more precise methods for measuring jitter. Jitter refers to the presence of abrupt, spurious variations in the amplitude or frequency of a periodic signal. Jitter is a component of signa-to-noise ratio (SNR), which is an important parameter in assessing the performance of a data converter. Jitter causes deviations in the sampling time, which, in turn causes deviations in the value of the signal sample. Jitter becomes a major factor in the SNR at relatively high input frequencies, for example, the frequencies at which communications analog-to-digital converters (ADCs) operate, because even a sall deviation in the sampling time may result in a large deviation in the value of the signal sample. As the demand for intermediate frequency sampling increases, ADC manufacturers are faced with the task of measuring SNR at very high input frequencies. The SNR at high input frequencies may be measured using a test system with very low jitter. Even stat-of-the-art systems cannot meet the low jitter requirement. Alternatively, the SNR may be measured using test systems with jitter by measuring the jitter of the system and then calibrating out the jitter. Known methods and systems of measuring jitter, however, have not been completely satisfactory in terms of accuracy, presision, and speed. The overall jitter may be due to internal factors inside the data converter such as the aperature jitter of a sample and hold circuit, or to external factors such as the jitter of a clock source or the phase noise of an input wave generator. The present application relates to means to measure noise caused by clock, input signal and aperture jitter.
Clock jitter has previously been measured using one of several techniques.
A first technique is to observe the spectral spreading or smearing around the spectral components of the clock. This technique is mainly used for circuits which must avoid generating energy in unwanted frequency bands. An example would be a cellular phone frequency synthesizer, which must produce a very pure sine wave which does not fail FCC compliance tests. This technique is unable to relate spectral spreading to time jitter, in RMS seconds.
A second commonly used technique is to slowly sweep the strobe signal of a strobed comparator across the rising or falling edge of the clock, capturing thousands of 1/0 outputs from the comparator. On a rising edge, the 1/0 sequence would start out with all 0's and then transition to all 1's. The percentage of 1's and 0's at each sweep position are recorded into a mathematical array. Then the derivative of the array provides the jitter histogram, from which the standard deviation of the jitter can be measured. This technique is very accurate, but requires a long test time because of the thousands of 1's and 0's that must be collected.
A third commonly used approach is to directly digitize the time intervals of the clock using a time digitizer. This type of circuit generally uses a time stretcher, which charges a capacitor with a fixed DC current for the time period to be measured and then discharges the capacitor with a small fraction of the DC current. The time required to discharge the capacitor is much easier to measure than the original time interval. Achieving one-shot measurements in the 20 ps range using this technique is very difficult, however. It is also very expensive.
A fourth technique involves digitization of multiple samples of the same point on a steeply rising edge with a constant slope (i.e., a very fast ramp). In this case, the voltage noise is related to the jitter by a very simple dv/dt relationship. This technique suffers from two problems. First the slope of the rising edge must be calibrated accurately, which is difficult to do. Second, there is no way to extract the voltage noise inherent to the digitization process from the jitter noise, since they both appear as voltage noise.
In recently filed Texas Instruments Inc. applications the signal to noise measurement and jitter measurement has been done using two test setups and two data sets. In one application of Burns et al. entitled “Method and Apparatus to Measure Jitter” filed Jul. 28, 1998, Ser. No. 09/124,199 an innovative system and method for measuring jitter of a high speed Analog to Digital (A/D) converter. This Burns et al. application is incorporated herein by reference. Aperture jitter in a Sample and Hold circuit (S/H) or in an A/D converter introduces noise into the sampled signal. The noise is more extreme in areas of the input waveform that have a steep positive or negative slope. The preferred embodiment allows an easy and inexpensive way to measure aperture jitter in S/H and A/D circuits. The technique can also be adapted for measuring edge jitter in digital clock signals or in analog sine wave signals. This application uses two data sets with one slower than the other. A first sine wave of a first amplitude and frequency and a second sine wave of a second amplitude and frequency and connected to receive evenly spaced samples and a provessor to compute signal amplitude and noise of the first and second wave and process that determine jitter. In accordance with another embodiment Ser. No. 60/171,260 (TI29000) filed Dec. 15, 1999 of Kuyel entitled “Method and System for Measuring Jitter”, jitter is determined by the step of first estimating the overal jitter by using the analog-to-digital itself using Dual Histograms of an Undersampled Sinusoid (DHUS) method. The aperature jitter of the analog-to-digital converter is estimated by time differential sampling method. The signal to noise ratio (SNR) is computed using conventional Fast Fourier Transform (FFT) method. The noise contribution due to overall jitter is subtracted out and the noise contribution due to analog-to-digital's aperature jitter is added in to get the signal to noise ratio. The system includes a data converter that measures a signal to generate a first measurement set and a second measurement set, which are used to compute overall jitter. The data converter generates the first measurement set and the second measurement set by measuring the signal. The overall jitter is computed using the measurement sets. According to one embodiment, a system for measuring internal jitter is disclosed that includes a splitter that splits a signal into an input signal and a clock signal. The data converter measures the input signal to generate a first data set and a second data set, which are used to compute the internal jitter of the data converter. According to one embodiment, a method for measuring internal jitter is disclosed. A signal is split into an input signal and a clock signal. The data converter measures the input signal to generate the first data set and the second data set. The internal jitter is computed using the first data set and the second data set. The external jitter is computed from the overall and internal jitter. The signal-to-noise ratio of the data converter is computed from the external jitter. This Kuyel application is incorporated herein by reference.
It is desirable to provide a measurement of jitter using one test set up and one data base and better repeatability on jitter measurements and possible test time improvements depending on the test platform being used.